Piezo-actuator

ABSTRACT

The invention relates to a piezoactuator having a stack ( 1 ) of piezoelectric layers ( 2 ) that lie on top of one another, which are separated from one another by means of an electrode layer ( 3 ), in each instance, in which a stabilization element ( 4 ) is attached at a side surface of the stack ( 1 ), which fixes in place the height of the stack ( 1 ) at the side surfaces in such a way that this height does not change when an operating voltage is applied to the electrode layers ( 3 ). It is particularly advantageous if the stabilization elements ( 4 ) are designed as pieces of sheet metal, whereby they can be used for electrical contacting and for mechanical stabilization of the piezoactuator at the same time.

[0001] The invention relates to a piezoactuator having a stack of piezoelectric layers that lie on top of one another and that are separated from one another via an electrode layer.

[0002] Piezoactuators are known to include electrode layers that engage each other like combs. The electrode layers in each comb are electrically connected via contact layers that are arranged at the edge of the stack, and are produced from a silver firing paste. In a center region of the stack, where the electrode layers that belong to different combs overlap, the actuator expands when an electrical voltage is applied. In contrast to this, no such expansion takes place in the edge regions of the stack. This results in tensile stresses in the inactive edge regions of the stack.

[0003] The tensile stress resistance of ceramic, which is generally used as a piezoelectric material, is typically low. Cracks that occur at a border between the center region and the edge region can spread on a surface of the ceramic, thereby endangering the operability of the component. This is because electrode layers in the actuator must be electrically connected to one another, i.e., in contact, to supply voltage at side surfaces of the actuator. An interruption in this electrical connection between electrode layers endangers the voltage supply to the electrode layers. This is because resulting currents can cause resistive heating at a damaged contact location, which leads to thermal destruction of an existing residual conductive connection.

[0004] To prevent formation of cracks in the edge region of the actuator, a piezoactuator can be moderately pressure-stressed. In the case of known piezoactuators, such moderate stress is provided by clamping the stack into a tube spring. The tube spring presses the base surface and the cover surface, respectively, of the actuator against one another at a uni-axial pressure of approximately 850 N.

[0005] Moreover, to prevent a failure of electrode layers during interruption of the contact layers arranged at the edge of the stack, numerous individual wires are soldered onto the actuator at regular intervals.

[0006] Clamping the actuator into a tube spring and applying of a large number of solder sites onto the contacting layer are complicated processes and are costly.

[0007] It is therefore a goal of the present invention to provide a piezoactuator in which the tensile stress at the edge of the stack is reduced by simple means.

[0008] This goal is achieved, according to the invention, with a piezoactuator according to claim 1. Advantageous embodiments of the invention are evident from the other claims.

[0009] The invention provides a piezoactuator that has a stack of piezoelectric layers that lie on top of one another. Electrode layers are arranged between these piezoelectric layers. A stabilization element is attached at a side surface of the stack, which fixes in place the height of the stack at the side surfaces in such a way that the height does not change when an operating voltage is applied to the electrode layers.

[0010] The piezoactuator according to the invention has the advantage that the tensile forces that occur at the side surface of the stack when the actuator is in operation can be absorbed by a stabilization element. This eliminates the need for a complicated design, such as the above-described tube spring design.

[0011] Furthermore, the stabilization element is attached at the side surfaces of the stack, i.e., precisely at the critical point where tearing of the piezoelectric layers can cause the greatest damage. The stabilization element also provides for a compact structure.

[0012] Stabilization of the actuator can be further improved by arranging stabilization elements preferably on two opposite sides surfaces of the stack.

[0013] In an advantageous embodiment of the piezoactuator according to the invention, the stabilization element is designed as a piece of sheet metal that extends over the entire height of the stack. The design of the stabilization element as a piece of sheet metal allows a very simple and material-saving way of making the element stable. Also, the stabilization element can be adapted it to various actuator designs. It can be adapted, for example, by the material selected for the sheet metal or by the thickness of the sheet metal used. The mechanical stability of the stabilization element can thus be optimally designed with a minimum use of material. In this regard, it is particularly advantageous to use sheet metal made of a specific metal, such as copper sheets or even aluminum sheets.

[0014] It is furthermore particularly advantageous if the stabilization element is attached over the entire surface of a side surface of the stack. Because tensile forces that occur are distributed over the entire side surface of the stack at the same time, maximum stability is achieved with a minimal attachment force. At the same time, attachment of the stabilization element can occur via, e.g., soldering, gluing, or even welding.

[0015] A particularly advantageous configuration comprises a piezoactuator having an electrical contact layer on each of two opposite side surfaces of a stack, for contacting the electrode layers, and having a stabilization element that is attached over the entire surface of each contact layer. This piezoactuator has the advantage that the electrical contact layers can be effectively protected against cracks resulting from tensile stress on the piezoelectric layers via the stabilization elements. In this way, the reliability of the electrical contacts of the electrode layers is improved.

[0016] Furthermore, it is also advantageous to use the stabilization elements themselves to provide contact between the electrode layers, since the stabilization elements and their attachment to the contact layers are electrically conductive, and since the stabilization elements contact external electrical connectors.

[0017] The stabilization elements demonstrate sufficient mechanical strength and are therefore not destroyed by the mechanical tensile stresses, in contrast to contact layers that only serve as electrical contacts. The stabilization elements also allow the application of a reliable contact element, for example soldering on a flexible braid, whereby the solder location can be structured in such a solid form that it is not easily damaged even if external vibrations occur.

[0018] Furthermore, a piece of sheet metal provided with perforations can be used as the stabilization element. Sheet metal with perforations offers the advantage of a reduced use of material, i.e., it has a lower weight than a solid piece of sheet metal. Because of the perforations in the sheet, sheet metal with perforations can also be attached by soldering more easily than can a solid piece of sheet metal.

[0019] The invention is explained in greater detail below, using an exemplary embodiment and the related FIGURE.

[0020] The FIGURE shows a cross-sectional schematic of an exemplary piezoactuator according to the invention.

[0021] The FIGURE shows a piezoactuator having a stack 1 of piezoelectric layers 2 that lie on top of one another. Piezoelectric layers 2 comprise a piezoceramic containing 67% by weight Pb3O4, approximately 1% by weight Nd2O3, 21% by weight ZrO2 and approximately 11% by weight TiO2. However, the invention is not restricted to this type of piezoceramic, but rather can be implemented with any piezoelectric layers 2 that demonstrate suitable piezoelectric properties.

[0022] Stack 1 has electrode layers 3 that engage one another like combs, causing the piezoelectric effects of many piezoelectric layers 2 to add up. Electrode layers 3 contain a mixture of silver and palladium in a weight ratio between 90/10 and 70/30. Stack 1 is produced by sintering together electrode layers 3 and piezoelectric layers 2, which is why all metals or metal alloys that are stable at sintering temperatures are suitable for use as electrode layers 3. Electrode layers 3 made of copper are a particularly good option. Electrode layers 3 that comprise a comb, in each instance, are connected with one another in an electrically conductive manner via a contact layer 5 that is affixed on the outside of the stack 1. Contact layer 5 can be produced from a silver firing paste, for example.

[0023] Stack 1 has the shape of a block with a base area of 7×7 mm. Stack 1 has a height of 60 mm. When a suitable electric voltage is applied between contact layers 5, stack 1 expands in the direction indicated with the two arrows, specifically and preferably in a center region M, where electrode layers 3 overlap. In an edge region R, where electrode layers 3 do not overlap, stack 1 expands less. Tensile stresses occur in a critical transition zone between center region M and edge region R. These tensile stresses on piezoelectric layer 2 can have an effect all the way to the edge of stack 1. To prevent tearing of piezoelectric layers 2 at the edge of stack 1, a stabilization element 4 is affixed there, in each instance.

[0024] An aluminum sheet with a thickness of 2 mm, for example, can be used as stabilization element 4. Stabilization element 4 extends over the entire height of the stack 1. Stabilization element 4, together with an appropriate form of fastening such as soldering, fixes the height of the stack 1 in such a way that the height does not change even when the piezoactuator is in operation. This prevents the occurrence of cracks in the edge region R of piezoelectric layers 2.

[0025] Furthermore, the stabilization element 4 can be a metallically conductive sheet metal, and thus can also be used to provide electrical contact for electrode layers 3. This is done, for example, by electrically connecting stabilization element 4 with a fixed external connector 6 via a braid 7 made of copper wire.

[0026] The present invention is not limited to the exemplary embodiment shown, but rather is defined in its most general form by claim 1. 

1. Piezoactuator having a stack (1) of piezoelectric layers (2) that lie on top of one another, and electrode layers (3) that lie between them, wherein a stabilization element (4) is attached at a side surface of the stack (1), which fixes in place the height of the stack (1) at the side surfaces in such a way that this height does not change when an operating voltage is applied to the electrode layers (3).
 2. Piezoactuator according to claim 1, wherein the stabilization element (4) is a piece of sheet metal that extends over the entire height of the stack (1).
 3. Piezoactuator according to claim 2, wherein the stabilization element (4) is attached over its entire surface.
 4. Piezoactuator according to claim 3, wherein the stabilization element (4) is attached by means of soldering, gluing or welding.
 5. Piezoactuator according to claims 1 through 3, wherein an electrical contact layer (5) for contacting the electrode layers (3) is arranged on two opposite side surfaces of the stack (1), in each instance, and wherein a stabilization element (4) is attached over its entire surface on each contact layer (5).
 6. Piezoactuator according to claim 5, wherein the stabilization elements (4) and their attachment on the contact layers (5) are electrically conductive, and wherein the stabilization elements (4) are contacted with external electrical connectors.
 7. Piezoactuator according to claims 1 through 6, wherein the stabilization elements (4) are pieces of sheet metal provided with perforations.
 8. Piezoactuator according to claims 1 through 7, wherein a stabilization element (4) is attached on two opposite side surfaces of the stack (1), in each instance. 